Offshore systems which are employed in relatively deep water for well operations generally include a riser which connects a surface vessel's equipment to a blowout preventer stack on a subsea wellhead. The marine riser provides a conduit through which tools and fluid can be communicated between the surface vessel and the subsea well.
Offshore systems which are employed for well testing operations also typically include a safety shut-in system which automatically prevents fluid communication between the well and the surface vessel in the event of an emergency, such as loss of vessel positioning capability. Typically, the safety shut-in system includes a subsea test tree which is landed inside the blowout preventer stack on a pipe string.
The subsea test tree generally includes a valve portion which has one or more normally closed valves that can automatically shut-in the well. The subsea test tree also includes a latch portion which enables the portion of the pipe string above the subsea test tree to be disconnected from the subsea test tree.
If an emergency condition arises during the deployment of tools on coiled tubing, for example, the safety shut-in system is first used to sever the coiled tubing. In a typical safety shut-in system, a ball valve performs both the function of severing the coiled tubing and the function of shutting off flow.
Although somewhat effective, the use of ball valves to sever the coiled tubing has proven difficult with larger sizes of coiled tubing. Additionally, use of the ball valves to perform cutting operations can have detrimental sealing effects on the sealing surfaces of the valve. Specifically, the sealing surfaces can become scarred, reducing the sealing efficiency.
There exists, therefore, a need for an efficient coiled tubing cutter.